a) Field of the Invention
The present invention relates to piezoelectric ceramics and a method of manufacturing the same.
b) Description of the Related Art
Single crystals of xPb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 -(1-x) PbTiO.sub.3 (hereinafter called PZN--PT based single crystal) and Pb (Zn.sub.1/3 Nb.sub.2/3)O.sub.3 (hereinafter called PZN based single crystal) are known as piezoelectric ceramic materials.
It is known that these materials have a high piezoeletric modulus in the perovskite structure and are excellent piezoelectric ceramic materials. It has been long desired to stinter the raw material to manufacture piezoelectric ceramics having the perovskite structure.
However, in these piezoelectric ceramic materials, an impurity phase or pyrochlore phase is easily formed. Sintering under a high pressure is therefore effective in order to suppress the generation of the pyrochlore phase.
In a conventional method, therefore, PZN or PZN--PT based piezoelectric ceramic material is provisionally fired to form powders. The powders arc pressed to form a pressed powder mold. This pressed powder mold is sintered in a hot isostatic pressing (HIP) apparatus at a high pressure at a temperature of 800-1150.degree. C. to manufacture a piezoelectric ceramic body.
Two-step processes for fabricating perovskite PZN and PZN--PT piezoelectric ceramics are disclosed in Japanese Journal of Applied Physics Vol.30, No.2B, 1991, L298-L301. "Processing of Perovskite Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 by Hot Isostatic Pressing and Its Dielectric Properties" and Ferroelectrics. 1992, Vol.134. pp.133-138, "Piezoelectric Properties of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3 Prepared by HIP". respectively.
In the former, perovskite PZN was fabricated from reagent-grade starting powders of PbO, ZnO and Nb.sub.2 O.sub.3 by means of a two-step process. The first step was the formation of pyrochlore PZN under atmospheric pressure. The composition of the starting powders was stoichiometric PZN except that PbO content was increased by 5 wt % to the stoichiometric PbO to compensate for its loss during the heating process. The powders were mixed by ball-milling in deionized water with zirconia media for 16 hours. The slurry was dried and calcined in a covered alumina crucible at 850.degree. C. for 1 h to form pyrochlore PZN. The calcined powders were ball-milled in deionized water with zirconia media for 12 h and dried at 120.degree. C. for 24 hours. The second step was the conversion of pyrochlore PZN to perovskite PZN by applying high pressure at high temperature using HIP. Pyrochlore PZN powders were pressed as pellets (10 mm in diameter and 2 to 4 mm in thickness) under 100 MPa. Various pressures (100-200 MPa) were applied to the pellets at various temperatures (800-1150.degree. C.) under Ar atmosphere using a commercial HIP.
In the latter, the first step is the formation of pyrochlore PZN--PT powders under atmospheric pressure with 5 wt % excess PbO from reagent-grade starting powders of PbO, ZnO, Nb.sub.2 O.sub.3 and TiO.sub.2. The second step is the conversion-step of the pyrochlore PZN--PT to the perovskite PZN--PT by applying high pressure (150-200 MPa) at high temperature (1150-1200.degree. C.) using a commercial HIP.
In another known method of sintering PZN--PT based piezoelectric ceramic material at an atmospheric pressure, lead (Pb) is replaced by another element such as barium (Ba). For example, a fraction of lead is replaced by barium (Ba). A pressed powder mold made of this material is sintered at an atmospheric pressure to manufacture piezoelectric ceramics. These ceramics have a high piezoelectric modulus. The replacement or substitution of Pb by barium is usually 6 to 7 mol % or more.
It is disclosed in Japanese Journal of Applied Physics Vol. 11, No. 3, 1972, pp. 358-364. "Dielectric and Piezoelectric Properties in the Ternary System of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 --PbTiO.sub.3, that in the vicinity of Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3, two phases of perovskite (ferroelectric) and pyrochlore (non-ferroelectric) types are coexistant, and the planar coupling factor reaches 0.44, for the composition of 0.8 Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 --0.2Pb.sub.0.7 Ba.sub.0.3 (Zn.sub.0.1 Nb.sub.0.2 Ti.sub.0.7)O.sub.3.
The samples were prepared by using PbO, BaCO.sub.3, ZnO Nb.sub.2 O.sub.5 and TiO.sub.2 (&gt;99.5% purity). The batches were calcined in a platinum crucible at 900.degree. C. for 10-20 hours. The calcined ones were ground, pressed into disks (18 mm in diameter), then sintered at 1100.degree. C. for 3 hours. To obtain uniform ceramics, the cycle was repeated again, and finally sintered at 1100-1200.degree. C. for 2-3 hours, depending on the compositions. In the system containing Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 the sintering was also done at a lower temperature for a longer period than described above, at 960.degree. C. for 15 hours, in order to suppress the formation of the second phase.
Further, it is disclosed in Am. Ceram. Soc. Bull., 66[4] 704-711,1987 "Preparation of Lead-Based Ferroelectric Relaxors for Capacitors" that their high dielectric constants, broad maxima, and relatively low firing temperatures have made them promising candidate materials for multilayer ceramic capacitors. However, such materials are difficult to fabricate reproducibly without the appearance of a pyrochlore phase that can be detrimental to the dielectric properties. The kinetics and thermodynamics of the perovskite vs pyrochlore formation is also reviewed.
The conventional manufacturing method suppresses the generation or formation of the pyrochlore phase by using high pressures sintering. However, high pressure gas is enclosed or entrapped within the disconnected pores in a sintered body of the piezoelectric ceramic body. When the sintered body is taken out from the HIP apparatus and exposed to atmospheric pressure after the HIP process, the pores expand and the sintered body is destroyed. So, a fine and dense sintered body cannot be manufactured. The sintered body may be broken by itself because of the expansion and burst of the high pressure pores.
In the case of the method of manufacturing piezoelectric ceramics by substituting barium for lead, a self-destruction of a sintered body will not occur because the sintering is performed at the atmospheric pressure. However, the Curie temperature is lowered, hindering the use of the sintered body under a high temperature environment compared with no substitution of the Ba ion the Pb ion.